Beneficiation of magnetite concentrates by flotation



atented Nov. 6, 1945 BENEFICIATION OF MAGNETITE CONCEN- TRATES BY FLOTATION Fred D. De Vaney, Hibbing, Minn., assignor to Eric Mining Company, Hibbing, Minm, a corporation of Minnesota Application November 3, 1943, Serial No. 508,849

9 Claims.

The present invention relates to the art of freeing magnetite-bearing mineral particles from naturally associated silicious gangue mineral par- .ticles whereby to produce a concentrate low in eral particles and silicious particles disseminated throughout the formations. They generally contain substantially more than 15% of silica, along with other gangue material, and only 40% or usually less of iron. In such materials, the iron generally occurs as hematite, limonite, magnetite, martite and siderite, and the gangue minerals include quartz and one or more of the following: fayalite (weakly magnetic); amphibole (weakly magnetic); minnesotite (non-magnetic) and stilpnomelane (non-magnetic) of which the last three are hydrous magnesium iron silicates differing from each other in the relative percentages of the elemental components. While the term taconite is more or less indigenous to the Mesabi Range in northeastern Minnesota, the geographical distribution of the material itself is not confined thereto.

In subjecting a magnetic taconite or any other silicious magnetite-containing ore to magnetic separation it frequently transpires that the attraction of the electromagnets for the magnetite content of the ground material is so strong that middling grains containing only a small amount of magnetite are carried into the concentrate. In the majority of such cases, exceedingly fine grinding of the material customarily would be necessary in order to liberate all or practically all of the magnetite from the silica with which it is locked." Such fine grinding materially adds to the cost of processing the material, since the cost of grinding to 200 mesh is practically double that of grinding the ore to 100 mesh.

It is an object of the present invention to pro videan improved process of obtaining concentrates low in silica from such silicious magnetitebearing materials, which process minimizes the amount of fine grinding necessary.

According to the present invention, the silicious magnetite-bearing material (e. g., taconite) is ground to such an intermediate state of flneness (say, to mesh) that a fair concentrate can be obtained therefrom by magnetic separation, which state of fineness is or may be short of that necessary for substantially complete unlocking of all the fine iron mineral particles from the silica particles with which they are locked, and the resulting magnetic concentrate is'subjected to a froth flotation treatment for floating off free silicious particles and the leanest middling grains, that is to say, the locked magnetite-silica particles relatively low in iron content, from the particles richer in iron minerals, using therefor a suitable cationic flotation reagent. The resulting floated portion of the material, if of sufiiciently high grade to warrant its further working up, thereafter is reground to a greater fineness (i. e., a state of subdivisione. g., to, say, 200 mesh eifective for unlocking more of the fine iron mineral particles) for re-concentration by magnetic separation or magnetic separation and froth flotation of the cationic type. If, however, the floated portion is of too low grade to warrant its further working up, the same is wasted. The relatively non-magnetic" product rejected by the magnetic separator is either (a) further treated for the recovery therefrom of iron minerals in non-magnetic form, or else (b) discarded-4n the event further working up is economically unwarranted.

In carrying out the cationic froth flotation step, it was found that magnetite has a greater tendency to float than has hematite, this floating tendency on the part of magnetite being somewhat akin to, but of a lower order than, that of silica. It was discovered, however, that the magnetite can be depressed into the underflow very effectively, without hindrance to the flotation of the silicious gangue material, by the use of a suitable addition agent in the pulp. Thus, it was found that this desirable depression of the magnetite can be effected by treating the pulp with a small addition of sodium silicate. Other operable magnetite-depressing agents are the alkaliacting alkali metal compounds, e. g., caustic soda. This technique, which has utility not only in the present relation but also in other flotation processes involving the selective separation of magnetite from silicious gangue material, is described and claimed in U. S. application Serial No. 505,697, filed October 9, 1943, by this applicant.

By the process above described one achieves in a most economical manner a satisfactory division of the silicious magnetite-bearing material into a low silica concentrate and a highly silicious "tailing" substantially freed from magnetic iron mineral, without having to fine grind more than a minor part of the total charge of starting material.

It has been found that this improved process is applicable to any silicious magnetite-bearing are or ore material, including the aforesaid taconites and the magnetite ores of the Appalachian district in which latter ores the gangue minerals are primarily silicious. In the cases of some silicious magnetite ores the magnetic concentrates, even after grinding to 100 or 150 mesh, contain a considerable amount of magnetic iron silicates and also middling grains in which the magnetite particles are in an extremely finely disseminated state: in such cases it has been found advantageous to float such material away from the cleaner magnetite particles and to discard the former, becuse the rejection of iron silicate is highly desirable and because no reasonable amount of grinding will liberate most of the extremely finely disseminated magnetite particles in such middling grains.

In the following the invention will be rescribed in greater detail, the illustrative but non-limitative examrple embodying data obtained in carrying out the process on a Mesabi 'taconite. The appended drawings, in which Fig. 1 is a fiow sheet of a process embodiment of the process of the present invention, and

Fig. 2 is a flow-sheet illustrating a modification of the process, also illustrates the invention.

The starting material was a sample of Mesabi taconite containing about 45% of gangue material-mostly silica-and about 35.25% Fe as magnetite, hematite, martite, siderite and greenalite, of which minerals the first three were the major components. The starting material, crushed and ground to 100 mesh, was subjected, wet, to magnetic separation under conditions to yield (1) an initial magnetic concentrate containing 63.25% Fe and 9.43% silica and (2) a tailing product containing 10.24% Fe and 80.56% silica. The weight of the initial magnetic concentrate represented 47.18% of the total charge of starting material.

For effecting this magnetic separation step,- there was used a three drum Steil'ensen magnetic separator. Each drum was five inches wide and twelve inches in diameter. This separator is of the wet low intensity direct current type and is manufactured by the Jeffrey Manufacturing Company of Columbus, Ohio.

In carrying out the magnetic separation step, the ore which had been crushed to 100 mesh was fed to the separator at a uniform rate as a slurry containing approximately 50% solids. The rate of feed was 176.5 lbs. per hour or 35.4 lbs.per inch of cobber width. The magnetizing coils in the separator drew a total of 3.0 amperes with a D. C. voltage of 103. The separator drums revolved at 36 R. P. M. or at a peripheral speed of 113 feet per min. In this machine, the three drums are operated in series, each successive drum reconcentrating the product from the preceding drum so that a finished concentrate is made only by the third drum. Finished taillngs were made from all three drums. 1

This initial magnetic concentrate, after having been passed through a demagnetizing coil, was made up into an aqueous pulp of about 25% solids content, and the pulp, with no further grinding, was treated in a 500 g. mechanically agitated flotation cell wherein silicious middling 'amine) which amine had been rendered watersoluble by conversion into its hydrochloride. This flotation reagent was employed in an amount equivalent to 0.2 lb. per 1 ton (of 2240 lbs.) of feed, dry weight (1. e., the weight of the mineral content of the feed dried to constant weight at 105 0.). To assist in the development of froth there was used about 0.05 lb./ton of methyl amyl alcohol (a frother). The pulp circuit was maintained alkaline by the addition thereto of about 2.0 lbs/ton of sodium silicate (or 0.5 lb./ton of caustic soda). The presence of this ingredient was found to favor the selective flotation of silica and the silicious locked iron mineral particles from the magnetite, which latter normally tends to have flotation characteristics similar to those of silica. As is customary in the technique of cationic froth flotation the flotation reagent was added to the pulp in small increments. I

The underflow product from this froth flotation step was a concentrate containing 65.36% Fe and 6.80% silica, and represented 70.85% by weight of the cell feed. The flotation froth was made up mostly of magnetite-quartz middling grains containing 58.12% Fe and 15.16% silica.

This latter froth product was re-ground, in a ball mill, to --200 mesh, and re-passed over a magnetic separator, yielding (a) a tailing and (b) a second magnetic concentrate containing 66.45% Fe and 5.38% silica. This magnetic concentrate (b), whose weight represented 81.48% of the total weight of the reground froth prod not, was, as illustrated in Fig. 1 of the drawings, combined with the underflow product of the froth flotation step to give a composite (final) concntrate containing 65.63% Fe and 6.44% silica, which final concentrate represented 44.63% by weight of the original starting material.

By this procedure a low silica concentrate was achieved in an economical manner by segregat- .ing the middling grains and fine-grinding only that part of the initial magnetic concentrate which required such grinding.

Illustrative of cases wherein re-grinding and re-processing are not economically justifiable,

I but wherein there may be produced products con- 11.96% Fe and 74.88% silica.

The initial magnetic concentrate, after having been demagnetized, was made up with water into a pulp of about 25% solids, and said pulp was subjected to froth flotation treatment, in a 500 g. mechanically agitated flotation cell, using as cationic flotation means about 0.15 lb./ton of a reagent, produced by Armour 8: Company, known as AM-1120 and understood to consist substantially of mono-n-dodecylamine, and as frothing agent about 0.05 lb./ton of a frother, produced by E. I. du Pont de Nemours & Companv, knownas frother #60 and understood to consist mainly of higher alcohols (approx. carbons). The pulp circuit was made alkaline by the addition thereto of about 1.4 lbs./ton of sodium silicate. There were obtained (a) a froth product, containing considerable iron silicate minerals, analyzing 42.56% Fe, and (b) from the underflow a finished concentrate analyzing 65.24% Fe and 8.10% silica, the weight of which concentrate represented 86.24% of the feed to the flotation cell or 33.00% of the original crude ore.

Due to the fact that the iron content of the froth product occurred largely as iron silicates, it was not feasible to re-grind and re-concentrate the same, and hence the froth product was discarded to waste. Its rejection made possible the production of a concentrate of considerably higher grade than that from mere magnetic separation.

While the above specific examples recite use of higher molecular weight primary alkyl amines as flotation reagents, it has been found that both primary and secondary aliphatic amines, and mixtures thereof, and cationically-acting collectors generally are operable in the present process.

In connection with that phase or embodiment of the present invention which includes the steps of regrinding the froth product to finer particle size (for unlocking more of the fine iron mineral particles) and reconcentrating this more flnely ground material by magnetic separation, it is within the scope of the invention either (a) as illustrated in Fig. 1, to accept the resulting magnetic concentrate as flnal product, for that purpose combining the same with the underflow pr oduct of the froth flotation step, or (b) as indicated in Fig. 2, to process said resulting magnetic concentrate by subjecting the same, after de-magnetization, to a second cationic froth flotation treatment for rejecting silicious grains, the underflow from such second flotation treatment being combined with the underflow product of the first flotation treatment to constitute the flnal composite product.

I claim:

Process of separately recovering magnetic and locked magnetite-silicious gangue particles from a pulp mixture containing the same together with free silica and non-magnetic oxidic iron, which comprises subjecting said mixture to magnetic separation so as to remove therefrom free silica and non-magnetic oxidic iron, demagnetizing the residual mixture, subjecting the residual mixture to froth flotation using a cationic collector whereby the locked magnetite-silicious gangue particles are floated while the free magnetite particles remain in the underflow, removing the froth product rich in locked magnetitesilicious gangue particles, and recovering substantially clean magnetite from the underflow.

2. Process of recovering magnetic concentrates from flnely divided taconites and similar finely divided mixtures of magnetite, locked magnetitesilicious gangue particles, free quartz, magnetic I free quartz and the non-magnetic iron silicates,

non-magnetic iron oxides, demagnetizing the residual mixture, and removing from the demagnetized residual mixture, by a froth flotation operation involving the use of a cationic collector, the magnetic iron silicates and the locked magnetite-silicious gangue particles as a froth product.

3. Process of separating magnetite from ores containing the same together with locked magnetite-silicious gangue particles, free quartz, magnetic and non-magnetic iron silicates and non-magnetic iron oxides, which comprises grinding the ore to a particle size somewhat coarser than that required for complete liberation, separating the ground mixture by magnetic separation into a non-magnetic portion comprising free quartz, non-magnetic iron silicates and non-magnetic iron oxides and a magnetic portion containing free magnetite together with locked magnetite-silicious gangue particles and magnetic iron silicates, demagnetizing the magnetic portion, and subjecting the demagnetized magnetic portion to a froth flotation operation involving the use of a cationic collector whereby the. magnetite is separated as underflow product from a froth product containing locked magnetite-silicious gangue particles and magnetic iron silicates.

4. Process of separating magnetite from ores containing the same together with locked magnetite-silicious gangue particles, free quartz, magnetic and non-magnetic iron silicates and nonmagnetic iron oxides, which comprises grinding the ore to a particle size somewhat coarser than that required for complete liberation, separating the ground mixture by magnetic separation into a non-magnetic portion comprising free quartz, non-magnetic iron silicates and non-magnetic iron oxides and a magnetic portion containing free magnetite together with locked magnetite-silicious gangue particles and magnetic iron silicates, demagnetizing the magnetic portion, subjecting the demagnetized magnetic portion to a froth flotation operation involving the use of a cationic collector whereby the magnetite is separated as underflow product from a froth product containing locked magnetite-silicious gangue particles and magnetic iron silicates. further grinding the solids content of the froth product substantially to size of liberation of the magnetite, and subjecting the reground material to magnetic separation for the recovery of magnetite therefrom.

5. Process of beneflciating a silicious magnetite-bearing material which when ground to 100 mesh contains free magnetite particles, free silicious gangue particles, and locked magnetitesilicious gangue particles, which comprises subdividing the material to a particle size of the order of 100 mesh; separating the so-subdivided material, by magnetic separation, into a non-magnetic portion and a magnetic concentrate portion containing magnetite particles and silicious middling particles; thereafter subjecting the magnetic concentrate portion to a froth flotation treatment involving the use of a cationic collector whereby to obtain a froth product containing silicious middling particles and, from the underflow, a concentrate of substantially clean magnetite; further subdividing the solids content of the froth product by grinding the same to a particle size, of the order of -200 mesh, such that magnetite is unlocked from the silicious middling particles; and separating the resulting finely ground material, by magnetic separation, into a magnetic concentrate portion and a nonmagnetic portion.

6. Process of beneflciating a finely divided silicious magnetite-bearing material, which contains free magnetite particles, free silicious gangue particles and locked magnetite-silicious gangue particles, which comprises separating the material, by magnetic separation, into a nonmagnetic portion and a magnetic concentrate portion containing magnetite particles and locked magnetite-silicious gangue particles: thereafter subjecting the magnetic concentrate portion to a froth flotation treatment, involving the use of a cationic collector and in an alkaline pulp circuit, whereby to obtain a froth product containing locked magnetite-silicious gangue particles and, from the underfiow, a concentrate of substantially clean magnetite; further subdividing the solids content of the froth product by grinding the same to such particle size that magnetite is unlocked; and separating the resulting more finely subdivided material, by magnetic separation, into a magnetic concentrate portion consisting essentially of clean magnetite and a non-magnetic gangue portion.

7. Process of beneficiating a finely divided silicious magnetite-bearing material, which contains free magnetite particles, free silicious gangue articles and locked magnetite-silicious gangue particles, which comprises separating the material, by magnetic separation, into a nonmagnetic portion and a magnetic concentrate portion containing free magnetite particles and locked magnetite-silicious gangue particles; and thereafter subjecting the magnetic concentrate portion to cationic froth flotation treatment using an aliphatic amine collector, in an alkaline pulp circuit, whereby to obtain a froth product,

containing locked magnetite-silicious gangue particles and, from the underfiow, a concentrate of substantially clean magnetite.

8. Process of benefieiating a finely divided silicious magnetite-bearing material, which contains free magnetite particles, free silicious gangue particles and locked magnetite-silicious gangue particles, which comprises separating the material, by magnetic separation, into a nonmagnetic portion and .a magnetic concentrate portion containing free magnetite particles and locked magnetite-silicious gangue particles; and thereafter subjecting the magnetic concentrate portion, in the form of an aqueous pulp made alkaline by the addition of an alkaline-acting alkali metal compound, to cationic flotation treatment in the presence of an aliphatic amine as cationic collector, whereby to obtain 'a froth product containing locked magnetite-silicious gangue particles and, from the underfiow, a concentrate of substantially clean magnetite.

9. Process of beneficiating a finely divided silicious magnetite-bearing material, which contains free magnetite particles, free silicious gangue particles and locked magnetite-silicious gangue particles, which comprises separating the material, by magnetic separation, into a non-magnetic portion and a magnetic concentrate portion containing free magnetite particles and locked magnetite-silicious gangue particles; and thereafter subjecting the magnetic concentrate portion, in the'form of an aqueous alkaline pulp containing sodium silicate, to cationic flotation treatment in the presence of an aliphatic amine as cationic collector, whereby to obtain a froth product containing locked magnetite-silicious gangue particles and, from the underfiow, a concentrate of substantially clean magnetite.

FRED D. DE VANEY. 

